The broad objective of the proposed research is to define the mechanisms involved in the cellular regulation of microtubule assembly during the mitotic cycle and for neuronal differentiation. Studies will employ clonal lines of cultured neuroblastoma cells in which in vivo patterns of microtubule utilization during neurite outgrowth and mitotic spindle formation can be coordinately investigated. To determine the total microtubule protein pool available for assembly, synthesis and degradation of tubule protein during the cell cycle in differentiated and non-differentiated cells will be analyzed. The distribution and orientation of microtubules in cells undergoing differentiation and at various stages of the cycle will be examined, and the percentage of the total microtubule protein pool assembled into microtubules determined. Utilizing in vitro assays for the polymerization of microtubules in neuroblastoma cell extracts, it will be established whether the modification of tubule subunits, the formation of nucleating sites or the existence of specific molecules inhibitory or stimulatory to polymerization are factors governing the cellular control of microtubule assembly. The proposed research should be of general significance in establishing the basic mechanisms by which microtubule-dependent processes in cellular division and differentiation are regulated.